1. Field of the Invention
The invention relates to a device for measuring the spectral reflectance of surface reflection light in mirrors, filters, lenses and the like, employed in optical devices. The invention furthermore relates to a process for measuring the spectral reflectance.
2. Description of Related Art
In an optical device, such as an exposure device, a light irradiation device, optical elements are employed such as reflectors, various filters, or lenses. These optical elements often require that the spectral reflectance be measured. The expression xe2x80x9cspectral reflectancexe2x80x9d is defined as the reflectance factor of light of a certain wavelength. For example, in a mirror fabricated, with a vacuum evaporated film on its surface, to reflect light of only a certain wavelength, as well as a lens or a filter having an anti-reflection film, it is often necessary to measure the spectral reflectance to confirm that the optical properties of the vacuum evaporated film formed or the antireflection film correspond with the computed values for the optical properties.
A conventional device used to measure the reflectance factor of the reflection surface of a mirror, lens, or filter, is shown in Japanese utility model application JP 55-21088. FIG. 6 is a schematic cross section of the arrangement of the JP 55-21088 device for measurement of the reflectance factor. This device for measuring the reflectance factor includes a cage-like body 71, a light source part 72, and a light receiving part 73. The lateral cross section of the cage-like body 71 is pentagonal and the cage-like body 71 has an angled top plate 75 and a bottom plate 77 in which a light transmission opening 76 is formed.
One of the oblique walls of the angled top plate 75 is provided with the light source part 72, while the other oblique wall is provided with the light receiving part 73. The angle of the uppermost part of the angled top plate 75 is defined by the size of the crossing angle at which the optical axis La of the light source part 72 and the optical axis Lb of the light receiving part 73 cross one another on the surface S of the measuring object M. Leg 78, which is one of several legs, projects from the bottom plate 77 and adjoins the surface S.
The crossing angle between the optical axis La of the light source part 72 and the optical axis Lb of the light receiving part 73 is fixed according to the angle of incidence of the light incident on the surface S. For example, in the situation in which the surface S of the reflection surface of a mirror is used to reflect incident light with an angle of incidence of 30 degrees, it is necessary to measure the reflectance factor in the situation in which the light is incident with an angle of incidence of 30 degrees on the surface S. The crossing angle between the optical axis La and the optical axis Lb is therefore 60 degrees in the vicinity of the surface S.
The reflectance factor of a reflection surface is generally the ratio of the change of the reflectance factor to the difference of the angle of incidence which becomes greater as the angle of incidence of the light becomes greater. The above described crossing angle in a device for measuring the reflectance factor is normally fixed in the range from 0 degrees to 120 degrees for the following reasons:
Many practical reflectors are used with an angle of incidence of the light in the range from 30 to 60 degrees.
To determine the characteristic of the antireflection film of a lens or filter, a measurement is taken in the state in which the angle of incidence is 0 degrees.
In the light source part 72 there are a light source lamp 81 and a diffuser 83. The light source lamp 81 is a small halogen lamp. In the light receiving part 73 there are a lens 84 on which the light reflected by the surface S is incident and a light receiving apparatus 85 which consists of a photoelectric cell.
In this device for measuring the reflectance factor, the light from the light source lamp 81 of the light source part 72 is scattered by means of the diffuser 83 and then the light is emitted forward with an irradiance which is uniform in all directions and is emitted via the light transmission opening 76 of the bottom plate 77 of the cage-shaped body 71 onto the surface S. The light reflected by the surface S is incident again via the light transmission opening 76 on the light receiving part 73 and is projected via the lens 84 onto the light receiving surface of the light receiving apparatus 85. Based upon the radiance of the above described reflection light, the reflectance factor of the surface S is determined.
In the light receiving part 73 a state is implemented in which over the entire range of the solid angle, which is viewed from the light receiving surface of the light receiving apparatus 85 via the lens 84, there is the image of the diffuser 83 broadened with a uniform irradiance. The amount of the light received by the light receiving apparatus 85 is therefore independent of the shape of the measuring object, but is proportional only to the reflectance factor of the surface S.
When the reflectance factor is measured by the above described device, it is necessary to obtain a reference which is characteristic of this device for measuring the reflectance factor. Then using this reference, the reflectance factor is determined in the manner described below.
Determination of the reference is done before measuring the reflectance factor of the measuring object or after the measurement task. Specifically, a standard mirror with a known reflectance factor is used to determine the reference. Specifically, light is emitted onto the reflection surface of the standard mirror by means of the device of FIG. 6 which will measure the reflectance factor and the irradiance of the reflection light. In this manner, a reference is determined for device for measuring the reflectance factor of measuring object.
The reflectance factor of the surface S can be obtained by the same computation employed to compute the quotient (a/b) and the spectral reflectance xcex1 of the above described standard mirror. That is, for the surface S, the quotient (a/b) is obtained by dividing the value a which is the irradiance of the reflection light obtained with respect to the surface S of the actual measuring object, by the value b which is the irradiance of the reflection light from the standard mirror (reference).
For example, in the case in which the value a of the irradiance of the reflection light measured with respect to the surface S is 7 mW/cm2, and in which the value b of the irradiance of the reflection light is 10 mW/cm2 which was obtained from the standard mirror with a spectral reflectance xcex1 of 80%, the reflectance factor of the surface S is computed as follows:
(7/10)xc3x9780(%)=56(%)
In the above described device for measuring the reflectance factor the disadvantages are the following:
(1) With respect to the measurement wavelength, when the reflectance factor is measured, it is necessary for the measurement light emitted onto the surface of the measuring object to have the same wavelength as the light which optically treats the above described measuring object for the actual application.
For example, in the situation in which the reflectance factor of an optical element such as a mirror used in a UV exposure device is measured, the desired result cannot be obtained when the reflectance factor of the UV light, with the same wavelength as the UV light which is intended to optically treat the optical element, is not measured. The wavelength of the light for treatment of the optical element can be different depending on the intended use of the optical device. For example, the light can be over a certain UV wavelength range or, alternatively, in a device for exposing a circuit pattern, the light can be a strictly predetermined wavelength of 365 nm which corresponds to the wavelength at which the resist has sensitivity.
It is therefore necessary to confirm that the reflectance factor of light in a certain UV range is high and is low with respect to light in the visible range and infrared range in order to check the optical characteristic of an mirror provided with a vacuum evaporated film used to reflect only the UV light while transmitting the visible radiation and the infrared light. Furthermore, with respect to a lens or a filter which is provided with an antireflection film it is necessary to confirm that the reflectance factor of the light is low over the entire wide wavelength range.
In a conventional device for measuring the reflectance factor, a process is utilized in which the light source lamp 81 is a halogen lamp and a photoelectric cell is used as the light receiving apparatus 85. Therefore, only the reflectance factor for all the light is measured over a wide wavelength range with the center consisting of the visible range, and the spectral reflectance, which is the reflectance factor of light within a certain wavelength, cannot be measured. Since a halogen lamp in the UV range does not have a high emission intensity, the reflectance factor in the UV range cannot be measured.
The above described example, in which the amount of light received by the light receiving apparatus 85 is proportional only to its reflectance factor regardless of the shape of the surface S, means a state exists in which, over the entire range of the solid angle viewed by the light receiving apparatus 85 that receives the reflection light, a reflection surface with uniform irradiance is formed. To form one such reflection surface with a uniform irradiance it is necessary for the diffuser 83 to form a surface light source with a uniform radiance.
FIG. 7 shows a schematic of the action of an ideal diffuser P1. Regardless of the direction of the incident light L1 the transmitted light L2 is uniformly scattered at the respective site on this diffuser P1 in all directions. This diffuser P1 accomplishes a state in which a surface light source with uniform irradiance is formed when viewed from the position in front of the exit side of the light as is shown by arrow V.
Plates of materials, such as opal glass or the like, are known as diffusers having this ideal light scattering function in the visible range and in the infrared wavelength range. They do absorb some light. But since a halogen lamp, in the above described wavelength range, has a high radiation intensity in practice there is no problem.
On the other hand, when a xenon lamp is used as the light source lamp to obtain light in the UV range, and if an attempt is made to take measurements only with respect to certain wavelengths, the radiation intensity becomes low. If a diffuser of a material which absorbs UV light is used, the intensity of the reflection light from the surface is reduced to the xe2x80x9cnoise levelxe2x80x9d. Therefore, there are situations in which the measurement becomes impossible.
In light of this circumstance, it is necessary to use a diffuser of a material which has high transmission factor with respect to UV light and can also scatter UV light. However, at present, with the exception of quartz frosted glass, there is no diffuser with this property.
Unfortunately, frosted quartz glass does not have sufficient light scattering ability. As shown in FIG. 8, in the situation in which a diffuser P2 of frosted quartz glass is employed the directions of primary scattering differ when the incident light L1 is scattered light. Therefore, a surface light source with an irradiance which has high uniformity in all directions cannot be obtained over the entire surface of the diffuser P2 when viewed from a position in front (direction of the arrow V). As a result the measurement of the reflectance factor cannot be made with high precision.
2) With respect to reduction in size, the above described prior device for measuring the reflectance factor has a structure in which a reduction in size is intended. However, the cross section of its cage-like body 71 has a large pentagonal shape, and the upper region in which the light source part 72 and the light receiving part 73 are located has a great width. If the spectral reflectance of the reflection surface of a small oval focusing mirror, with a small radius of curvature, is measured there are situations in which the top wide area of the cage-like body 71, the wide light source part 72, or the wide light receiving part 73 partially interfere with the measuring object when the radius of curvature of the bent measuring object M is smaller than in the state shown in the drawings using the broken line. There are therefore also cases in which the bottom plate 77 of the above described measurement device which is provided with the light transmission opening 76 is not in the proper position and an exact measurement cannot be made.
3) With respect to obtaining the reference, in the above described device for measurement of the reflectance factor it is necessary to make available a standard mirror with a known reflectance factor in order to obtain a reference. The standard mirror must be carefully stored and handled so that its spectral reflectance does not change over time. However, if the spectral reflectance of the standard mirror changes, the change cannot be determined, thereby resulting in the consequence that errors will arise in the measurement of the spectral reflectance of the measuring object.
The invention was provided to eliminate the above described defects in the prior art. The object of the invention is to provide a device for measurement of the spectral reflectance in which light in the UV range can be used as measurement light, uniform irradiance can be obtained in a diffuser, and the reflectance factor can be measured in a wide wavelength range, including the UV range, by means of measurement light with a certain wavelength.
Another object of the invention is to provide a device for measurement of the spectral reflectance in which the head part adjoining the measuring object can be reduced in size to a sufficient degree such that even when using an oval focusing mirror with a small radius of curvature, the spectral reflectance of the reflection surface on its inner mirror surface can be measured.
Still another object of the invention is to provide a device for measurement of the spectral reflectance in which the reference can be easily obtained without using a standard mirror with a known spectral reflectance.
Still another object of the invention is to provide a process for measurement of the spectral reflectance by means of the above described device for measurement of the spectral reflectance.
These objects are achieved in a device for measurement of the spectral reflectance by providing the following features:
a light source part with a xenon lamp;
an optical fiber on the incidence side which transmits light from the above described light source part;
a measurement head which emits the light, transmitted by the optical fiber on the incidence side via a convergent lens and a diffuser, onto the surface of the measuring object and receives the light reflected by the above described surface;
an optical fiber on the exit side which transmits the reflection light which has traveled through the measurement head; and
a spectroradiometer which receives the light which has been transmitted by the optical fiber on the exit side.
Within the framework of the invention the expressions xe2x80x9coptical fiberxe2x80x9d or xe2x80x9cfiberxe2x80x9d designates both a single fiber and a bunch of fibers consisting of several fibers.
The objects of the invention are further achieved in the above described device for measurement of the spectral reflectance in an embodiment in which measurement head has a light incidence holder and a light emergence holder which is independent and separate of the light incidence holder. The light incidence holder has on one end a fiber connection part to which the fiber on the incidence side is connected; while, on the other end of the light incidence holder a light transmission opening is provided. Proceeding from the side of the fiber connection part, the convergent lens and the diffuser are located in sequence in the light incidence holder. In the light emergence holder, there is a lens for receiving the light reflected by the surface situated between one end which is located opposite the surface S, and the other end which is provided with the fiber connection part to which the fiber is connected on the exit side. Furthermore, the light incidence holder and the light emergence holder are coupled in one piece in the manner to be separable from one another so that the respective optical axes on the surface S of the measuring object M or in the vicinity thereof cross each other.
The objects of the invention are furthermore achieved in an embodiment in which in the light emergence holder of the measurement head there is a mirror which reflects the light reflected from the surface such that the direction in which the light emergence holder extends is essentially identical to the direction in which the light incidence holder extends, such that their axes are substantially parallel.
The objects of the invention are furthermore achieved in an embodiment in which the light incidence holder and the light emergence holder of the measurement head can be coupled to one another in such a manner that the optical axes of the two holders agree with one another, and such when in such a coupled state, with respect to the reference light, a reference light measurement can be taken. In this embodiment, an arrangement can be provided in which the light incidence holder and the light emergence holder can be coupled to one another by means of a template for reference light measurement.
The objects of the invention are furthermore achieved by a process for the measurement of the spectral reflectance using the above described device for measurement of the spectral reflectance where the light incidence holder and the light emergence holder are coupled to one another in a manner in which the optical axes of the two are in agreement, i.e., substantially parallel, with each other so that the reference light is measured, and where the light incidence holder and the light emergence holder of the measurement head are coupled to one another in a manner in which the respective optical axes cross on the surface of the measuring object or in the vicinity such that the spectral reflectance of the measuring object surface is measured. Therefore, based upon the relation between the spectral irradiance of the reference light and the spectral irradiance of the reflection light, the spectral reflectance of the surface is determined.
The objects of the invention are furthermore achieved by a device for measurement of the spectral reflectance by the following features:
a light source part with a xenon lamp;
an optical fiber on the incidence side which transmits light from the above described light source part;
a measurement head which emits the light, transmitted by the fiber on the incidence side via a convergent lens, a diffuser, and a semi-transparent mirror, onto the surface of the measuring object and moreover which receives the light reflected by the above described surface via the semi-transparent mirror; and
an optical fiber on the exit side which transmits the reflection light which has traveled through the measurement head; and
a spectroradiometer which receives the light which has been transmitted by the above described fibers on the exit side.
The device for measurement of the spectral reflectance of the above described arrangement can employ a xenon lamp as the light source lamp, and, moreover, a spectroradiometer for determining the illuminance of the reflection light. Therefore, the spectral reflectance can be measured at the respective wavelength in a wide range of wavelengths from the UV range, upto the visible and the infrared range.
Furthermore, the light source part and the measurement head are optically connected to one another via the optical fibers. The measurement head and the spectroradiometer are similarly optically connected to one another via the optical fibers. The degree of versatility with respect to the given arrangement of the light source part, the measurement head, and the spectroradiometer is therefore great.
Additionally, the measurement head on the invention can be made to have a smaller configuration since when measuring the spectral reflectance it is satisfactory to move only the tip area of the small measurement head close to the surface, such that measurement of the spectral reflectance for a focusing mirror with a small radius of curvature and in a narrow range is possible.
It is noted that the reference can be easily obtained directly by means of the above described device for measurement of the spectral reflectance when the measurement head is reassembled into the state for reference light measurement. As a result, a standard mirror with a known reflectance factor is not needed. Furthermore, in the conventional situation for reference measurement using a standard mirror the change of the reflectance factor of the standard mirror can also be measured.